In aircraft, compressors are provided in association or integral with gas turbine engines in order to compress air for engine use and/or supply the compressed air to various apparatus in the aircraft for use thereby. For example, compressed air is supplied to the cabin of the aircraft to pressurize the cabin and supply cooled air thereto. The compressor is driven directly by the driveshaft of an aircraft engine or through a transmission device or the like.
Typically, such a compressor, in association with an aircraft engine, supplies more compressed air than is needed by the apparatus in the aircraft which makes use of the compressed air. The excess compressed air output by the compressor is supplied to a dump tube for dumping the excess compressed air to avoid compressor surge. Surges in the compressor occur during start-up of the aircraft engine and also when the demand for compressed air decreases due to the sudden decrease in operation of apparatus which makes use of the compressed air output by the compressor.
Various conventional apparatus have been proposed for eliminating surges in the compressed air output by a compressor during start-up of the compressor and during sudden decrease in operation of apparatus which makes use of the compressed air output by the compressor.
For example, U.S. Pat. No. 1,215,071 discloses an unloading valve for a compressor which automatically unloads a portion of the compressed air output from the compressor whenever the compressor stops its compressing action. Thus, the unloading valve disclosed by U.S. Pat. No. 1,215,071 facilitates the start-up of the compressor against pressure in the main lines or tank beyond the unloading device to which compressed air from the compressor is supplied.
The unloading valve disclosed by U.S. Pat. No. 1,215,071 suffers from the disadvantage of not providing apparatus which aids in eliminating surges in the compressed air under low flow conditions in which surge may occur during operation. This concept is only utilized when the compressor is shut down or inoperative.
U.S. Pat. No. 2,459,000 discloses a spill valve which is operated in response to a Venturi throat for controlling the pressurization of an aircraft cabin during periods of low altitude flying. Particularly the spill valve disclosed by U.S. Pat. No. 2,459,000 is used to dump compressed air during the periods of low altitude flying.
The apparatus disclosed by U.S. Pat. No. 2,459,000 suffers from the disadvantage of not providing apparatus for eliminating compressor surges during the start-up and operation of the compressor. Further, the apparatus disclosed by U.S. Pat. No. 2,459,000 does not disclose apparatus for reducing compressor dump flow when additional apparatus which makes use of the compressed air is suddenly started or when the operation of apparatus which makes use of the compressed air is increased while maintaining compressor surge margin.
U.S. Pat. No. 2,977,971 discloses a fluid distribution system and valves having an unloading valve interposed between a constant displacement pump and a distribution line. The valve disclosed by U.S. Pat. No. 2,977,971 is designed to prevent damage to the pump when pressure surges are encountered in the system. The unloading valve disclosed by U.S. Pat. No. 2,977,971 rapidly shifts between its loading and unloading position so that the flow of unloaded fluid is not throttled or graduated.
The fluid distribution system and valves disclosed by U.S. Pat. No. 2,977,971 suffers from the disadvantage that the apparatus disclosed thereby does not provide apparatus for eliminating surges during the start-up and operation of the compressor. Further, the fluid distribution system and valves disclosed by U.S. Pat. No. 2,977,971 suffers from the disadvantage of not providing apparatus for eliminating compressor surges while reducing the dumped air quantity when the system demand for compressed air has increased by either the start-up of additional apparatus which makes use of the compressed air or the increased operation of the apparatus which makes use of the compressed air.
U.S. Pat. No. 3,470,896 discloses a flow control system having main and base load pipes connected in parallel between a source of fluid supply and a discharge wherein the base load pipe has an orifice in which the difference in pressure between the high pressure side and the low pressure side of the orifice is used to control a valve in the main pipe thereby regulating the flow in the main pipe.
U.S. Pat. No. 3,470,896 suffers from the disadvantage of not providing apparatus which eliminates compressor surges while reducing the dumped air quantity when the system demand for compressed air during the start-up and operation of the compressor has increased. Further, the fluid flow control system disclosed by U.S. Pat. No. 3,470,896 suffers from the disadvantage of not providing apparatus which eliminates compressor surges while reducing the dumped air quantity when the system demand for compressed air has increased by either the start-up of additional apparatus which makes use of the compressed air or the increased operation of the apparatus using the compressed air.
U.S. Pat. No. 3,698,839 discloses a pressure equalizer for unloading a compressor during start-up having a Venturi throat valve which initially operates with equalized pressure across the Venturi throat valve so that back pressure is present on the compressor during start-up. The pressure differential across the Venturi throat valve of U.S. Pat. No. 3,698,839 is developed in the discharge line by a spring-bias axially movable nozzle which is movable in opposition to its spring bias and by a force resulting from the pressure differential to an operating position effective to close the vent and interrupt the equalization passage.
The pressure equalizer disclosed by U.S. Pat. No. 3,698,839 suffers from the disadvantage of not providing apparatus which eliminates compressor surge during the start-up and/or compressor operation. This device is for simply unloading the compressor during start-up and until a minimum flow is obtained. The intent of the pressure equalizer disclosed by U.S. Pat. No. 3,698,839 is to reduce motor sizing and not to prevent compressor surges.